1. Field of the Invention
The present invention relates to an organic electroluminescence (EL) cell and to a method for producing the same. More particularly, the present invention relates to an organic EL cell that retains its stable light-emitting characteristics over a long period of time and to a method for producing such an organic EL cell.
2. Description of the Related Art
The structure of a conventional organic EL cell is shown in FIG. 4. The organic EL cell includes a substrate 1 that is transparent to light and a laminate structure 8 that is formed over the substrate 1. The laminate structure 8 includes an anode 2, a hole injecting layer 3 that is made of an organic compound, a hole transporting layer 4, a light emitting layer 5, an electron injecting layer 6, and a cathode 7, all of which are formed over one another as shown in FIG. 4.
The substrate 1 is made of, for example, a transparent glass, quartz, sapphire, or an organic film. The anode 2 is made of, for example, indium tin oxide (ITO). The cathode 7 is made of a highly reactive alloy composed mainly of an alkali metal or an alkaline earth metal, which is capable of electron emission at a low energy level.
The disadvantage of the organic EL cell shown in FIG. 4 is that the laminate structure 8 is exposed to the atmosphere. As a result, the cathode 7 of the laminate structure 8 is subject to oxidation by the moisture contained in the atmosphere. This oxidation by the moisture leads to deterioration in the light-emitting characteristics of the organic EL cell.
Several arrangements have been contemplated to overcome this disadvantage. For example, Japanese Patent Laid-Open Application No. 09-148066 discloses the sealing of the laminate structure 8 in an airtight container 10, which holds a desiccant 9, as shown in FIG. 5A.
A second arrangement is disclosed in Japanese Patent Laid-Open Application No. 08-111286. The second arrangement suggests covering the surface of the laminate structure 8 with a protective film 11, which is made of SiO2 or Si3N4, as shown in FIG. 5B. Still, a third arrangement disclosed in Japanese Patent Laid-Open Application No. 10-144468 suggests covering the surface of the laminate structure 8 with a resin film for isolation from the atmosphere.
Each arrangement has a respective drawback. The first one increases the thickness and the weight of the EL cell. The second one needs a thick protective film and consequently, increases residual stress and takes a longer time for its formation. The residual stress may cause warpage and crack the substrate and thus, degrade characteristic properties of the EL cell. Moreover, the protective film has to be thick enough to protect the EL cell from damage during the manufacture process or the repair process, which includes a step of burning or opening a defective cell with a laser beam. Finally, the third arrangement is incomplete in preventing the cell from moisture.